This invention relates to a process of producing polybutylene terephthalate (PBT) from butanediol (BDO) and terephthalic acid (TPA) or from butanediol (BDO) and dimethyl terephthalate (DMT), in which a mixture of BDO and TPA or BDO and DMT is subjected to an esterification or transesterification and subsequently to a polycondensation, in the presence of a catalyst solution containing alkyl titanate at temperatures in the range from 130 to 260° C.
It is known that PBT is produced from TPA or DMT by reaction with BDO by using catalysts. From among the multitude of catalysts used, in particular alkyl titanates, preferably tetrabutyl orthotitanate and tetraisopropyl orthotitanate, have gained acceptance due to their efficiency and large-scale availability (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A 21, page 237). The alkyl titanates have properties which have a disadvantageous effect in the production of PBT, because they tend towards hydrolysis and in particular in the process of producing PBT on the basis of TPA form insoluble particles during esterification, which particles not only reduce the service life of the filters for the PBT melt, but also impair the quality of the PBT produced. As a result of their volatility, thermal decomposability and easy reducibility, alkyl titanates can also form titanium-containing dark deposits at the reactor walls and in the process column, which impair the quality of the PBT produced.